The present invention generally relates to a system for stimulating a Hypoglossal nerve for controlling the position of a patient's tongue. In one embodiment, the Hypoglossal nerve is stimulated to prevent obstructive sleep apnea.
Sleep apnea is a sleep disorder characterized by pauses in breathing during sleep. Those affected by sleep apnea stop breathing during sleep numerous times during the night. There are two types of sleep apnea, generally described in medical literature as central and obstructive sleep apnea. Central sleep apnea is a failure of the nervous system to produce proper signals for excitation of the muscles involved with respiration. Obstructive sleep apnea (OSA) is caused by episodes of physical obstruction of the upper airway channel (UAW) during sleep. The physical obstruction is often caused by changes in the position of the tongue 110 during sleep that results in the closure of the soft tissues at the rear of the throat or pharynx (See FIGS. 1, 2A and 2B).
OSA is characterized by the complete obstruction of the airway causing breathing to cease completely (Apnea) or partially (Hypopnea). The human airway (at the level of the thorax) is lined by soft tissue, any collapse of its walls results in the closure of the airway which leads to insufficient oxygen intake, thereby interrupting one's sleep (episodes or micro-arousals).
During sleep, the tongue muscles relax. In this relaxed state, the tongue may lack sufficient muscle tone to prevent the tongue from changing its normal tonic shape and position. When the base of the tongue and soft tissue of the upper airway collapse, the upper airway channel is blocked, causing an apnea event (FIG. 2B). Blockage of the upper airway prevents air from flowing into the lungs, creating a decrease in blood oxygen level, which in turn increases blood pressure and heart dilation. This causes a reflexive forced opening of the upper airway channel until normal patency is regained, followed by normal respiration until the next apneaic event. These reflexive forced openings briefly arouse the patient from sleep.
OSA is a potentially life-threatening disease that often goes undiagnosed in most patients affected by sleep apnea. The severity of sleep apnea is determined by dividing the number of episodes of apneas and hypopneas lasting ten seconds or more by the number of hours of sleep. The resulting number is called the Apnea-Hypopnea Index, or AHI. The higher the index the more serious the condition. An index between 5 and 10 is low, between 10 and 15 is mild to moderate, over 15 is moderately severe, and anything over 30 indicates severe sleep apnea.
Current treatment options range from drug intervention, non-invasive approaches, to more invasive surgical procedures. In many of these instances, patient acceptance and therapy compliance is well below desired levels, rendering the current solutions ineffective as a long-term solution.
Current treatment options for OSA have not been consistently effective for all patients. A standard method for treating OSA is Continuous Positive Airway Pressure (CPAP) treatment, which requires the patient to wear a mask through which air is blown into the nostrils and mouth to keep the airway open. Patient compliance is poor due to discomfort and side effects such as sneezing, nasal discharge, dryness, skin irritation, claustrophobia, and panic attacks. A surgical procedure where rigid inserts are implanted in the soft palate to provide structural support is a more invasive treatment for mild to moderate cases of OSA. Alternate treatments are even more invasive and drastic, including uvulopalatopharyngoplasty and tracheostomy. However, surgical or mechanical methods tend to be invasive or uncomfortable, are not always effective, and many are not tolerated by the patient.
Nerve stimulation to control the position of the tongue is a promising alternative to these forms of treatment. For example, pharyngeal dilation via Hypoglossal nerve (XII) (FIG. 3) stimulation has been shown to be an effective treatment method for OSA. The nerves are stimulated using an implanted electrode to move the tongue and open the airway during sleep. In particular, the medial XII nerve branch (i.e., in. Genioglossus), has demonstrated significant reductions in UAW airflow resistance (i.e., increased pharyngeal caliber). While electrical stimulation of nerves has been experimentally shown to remove or ameliorate certain conditions (e.g., obstructions in the UAW), current implementation methods typically require accurate detection of a condition (e.g., a muscular obstruction of the airway or chest wall expansion), selective stimulation of a muscle or nerve, and a coupling of the detection and stimulation. These systems rely on detection of breathing and/or detection of apnea events as pre-conditions to control and deliver electrical stimulation in order to cause only useful tongue motions and to periodically rest the tongue muscles and avoid fatigue. In one system, for example, a voltage controlled waveform source is multiplexed to two cuff electrode contacts. A bio-signal amplifier connected to the contacts controls stimulus based on breathing patterns. In another system, a microstimulator uses an implanted single-contact constant current stimulator synchronized to breathing to maintain an open airway. A third system uses an implantable pulse generator (IPG) with a single cuff electrode attached to the distal portion of the Hypoglossal nerve, with stimulation timed to breathing. This last system uses a lead attached to the chest wall to sense breathing motions by looking at “bio-impedance” of the chest wall. Still another system monitors vagus nerve electroneurograms to detect an apnea event and stimulate the Hypoglossal nerve in response.
What is needed is a system and method of electrical stimulation of the Hypoglossal nerve for controlling tongue position that is not tied to the detection of breathing and/or an apnea event.